This invention relates to an apparatus and method for measuring a property of a liquid. More particularly, it relates to the characterisation of liquids by obtaining fingerprints for the liquids, from which various properties can be derived.
There is a need for a range of techniques to analyse liquids to detect differences between samples, such as in quality control, detection of counterfeit goods, or analysis of tampering. The invention also has as its aim the provision of an improved technique for use in forensic analysis of liquids.
According to the present invention there is provided an apparatus for use in measuring a property of a liquid comprising means for directing acoustic energy at a sample of the liquid, and means for deriving a signal related to the interaction of the acoustic energy with the liquid sample.
The invention further provides a method for measuring a property of a liquid comprising directing acoustic energy at a sample of the liquid, deriving a signal related to the interaction of the acoustic energy with the liquid sample, and determining a property of the liquid based upon the said signal.
The liquid, which is preferably in the form of a drop, may be a pure liquid, a mixture of liquids, a solution of a solid substance, a suspension of a solid within a liquid or a colloidal suspension, and the term xe2x80x9cliquidxe2x80x9d is used herein to signify all of these possibilities.
The term xe2x80x9cdropxe2x80x9d, as used herein, is defined as the interface that is formed between a liquid and any other material or materials, such as a solid, gas or liquid, under the force of the surface tension of the interface usually, but not always, combined with the force of gravity, acting on a particular geometrical arrangement of said liquid and materials.
The drop can be formed using bioactive agents or radioactive material.
The drop can undergo one or more phase changes during or at a particular point in its growth cycle, and it can be contaminated by, or dissolved, or evaporated into a surrounding medium.
Acoustic energy may be launched at the liquid sample from one or more sources, and the signal produced by the interaction with the sample can be obtained by various methods. One method is by examination of the loading effect on the driving signal to the acoustic source(s). A second is by comparing the driving signal to the modulated signal received by one or more detectors. The formation of the drop can take place directly upon an acoustic source or at the end of an acoustic guide attached to a source and/or a detector. Both configurations will henceforth be referred to as an xe2x80x9cacoustic drop headxe2x80x9d. The drop head can be arranged to form either pendant or sessile drops.
The drop formed at the end of the acoustic drop head can be analysed statically for a fixed drop volume or dynamically during its growth and release cycle, where one or more of a succession of drops are allowed to drip from the end of the acoustic drop head. Thus one may analyse small volumes, or continuously monitor a liquid by bleeding off sample droplets for analysis. The liquid may also be allowed to evaporate from the drop formed under both static or dynamic drop conditions.
The acoustic energy may be any acoustic radiation that can be coupled into the drop, and it can be pulsed, continuous, varying in frequency, amplitude or phase or otherwise modulated to facilitate the particular analysis being performed. The acoustic energy can be coupled directly into the drop using the acoustic drop head, which is in direct contact with the test liquid or alternatively it can be launched at the unattached surface of the drop through a medium of different acoustic impedance. In such a case one may monitor two or more signalsxe2x80x94one is the signal that penetrates the drop, and the other is based on the acoustic field set up immediately outside of the drop. In some embodiments the source transducer may also act as the detector transducer.
The signal related to the interaction of the acoustic energy and the liquid sample can be measured using an acoustic transduction measurement system. Any change in the driving signal from the acoustic radiator and the modulated signal is then used to deduce properties relating to the liquid under examination. The changes in the electrical driving signal referred to above may arise from variations in the acoustic impedance or geometry of the test liquid, thereby xe2x80x9cloadingxe2x80x9d the signal produced by the source transducer. Changes in the acoustic signal may affect amplitude, phase, frequency, reverberation time, harmonics etc.
The invention can be used to measure indirectly the following properties of a liquid:
Surface Tension
Viscosity
Acoustic Impedance (i.e. density and the speed of sound within the test liquid).
From these measurements, other properties of the test liquid ban be determined, such as the concentration level of other species contained within the liquid.
The acoustic energy may be optimised in terms of frequency and power to set the drop of liquid into oscillation. Devices that monitor the oscillation of the drop that is energised in this fashion and simultaneously or subsequently monitor the drop by its acoustic signal as described above are within the scope of this invention. In addition, the invention includes the acoustic analysis of drops that are stimulated into oscillation or movement by any other means, such as mechanical energy, acoustic energy, an external force, thermal energy or as the result of a chemical or biological reaction
Embodiments of this invention are also possible where the surface of the sample liquid is not a drop, but rather forms the meniscus of a liquid within a tube, preferably a capillary tube.